It sounds as though the oven control thermostat may be faulty, checking with a multimeter or meggar is the only way to be certain. If you don't have access to those or the know how to use them then it is time to call for the cavalry.
Please be sure to isolate from the mains if you intend to have a look.
NB. There are two different oven controls used on these models, full serial number is required to ID correct t parts.
That which can be asserted without evidence, can be dismissed without evidence.